This invention relates to automatic vehicle identification (AVI) systems used to detect a specific vehicle over an inductive loop embedded in a roadbed. More particularly, this invention relates to a technique for improving the performance and reliability of such systems.
AVI systems have been used for a substantial period of time to generate information specifying the presence or absence of a specific vehicle at a particular location sometimes termed a detection zone and to control access to restricted areas, such as an area providing restricted access via an automatically operated gate. Such systems have been used, for example, to limit ingress and egress at police impound lots to only authorized vehicles, to enable emergency vehicles (such as fire trucks and ambulances) to gain access to a gated residential or industrial area, and to monitor the progress of omnibuses along a city route.
A typical AVI system has a transmitter mounted on a vehicle which broadcasts over a limited range an encoded signal, usually a modulated carrier signal at a predetermined specific frequency (e.g., 375 kHz.), serving to identify the vehicle. A receiver connected to a loop antenna detects signals sensed at the specific frequency when a vehicle having such a transmitter is within the detection range of the loop-receiver combination. The receiver processes the detected signal to recover the encoded information, determines whether the encoded information matches a permissible code sequence stored in the receiver (which specifies a vehicle authorized in the system), and generates appropriate supervisory and control signals depending on the result of this determination. For example, in a system application in which access to a gate-guarded area is controlled by a receiver, the receiver may generate a gate operating signal in response to a match between the encoded information detected by the receiver and the permissible code sequence stored in the receiver. In a vehicle progress monitoring application, the receiver may time stamp the passage of the specific vehicle through the loop and store this time information and the identity of the vehicle in a local memory or transmit this information to a central processing unit, either instantaneously or on a periodical batch processing basis.
In a typical U.S. AVI system, information is encoded on a single frequency carrier signal by carrier burst modulation using a serial bit trinary encoding technique. According to this trinary encoding technique, a bit clock having a period of 0.1706 msec. Is used to define a bit period of 0.68250 msec. (four bit clock cycles); a trinary bit period of 1.3650 msec consisting of two consecutive bit periods. (eight bit clock cycles), and a nine bit trinary sequence of 12.285 msec (seventy-two bit clock cycles). The bit clock is also used to define two different types of bits—a short bit and a long bit. A short bit is defined as a binary signal asserted for the duration of one-half of a bit clock cycle (0.08530 msec.). A long bit is defined as a binary signal asserted for the duration of three and one-half cycles of the bit clock (0.59720 msec.). A ZERO value is defined as a two short bits during a trinary bit period consisting of a short bit at the beginning of a bit period followed by another short bit at the beginning of the next consecutive bit period. A ONE value is defined as two long bits during a trinary bit period consisting of a long bit at the beginning of a bit period followed by another long bit at the beginning of the next consecutive bit period. A TWO value is defined as one long bit followed by one short bit during a trinary bit period consisting of a long bit at the beginning of a bit period followed by a short bit at the beginning of the next consecutive bit period. In a nine bit trinary sequence, the least significant bit is transmitted first, followed by the next most significant bit, etc., until the most significant bit has been transmitted. The order of the bits is weighted according to the trinary encoding system, so that a transmitted value of ONE for the first trinary bit in the trinary sequence (bit 0) is interpreted as ONE, and a transmitted value of TWO for trinary bit 0 is interpreted as TWO; a transmitted value of ONE for the second trinary bit in the trinary sequence (bit 1) is interpreted as THREE, a transmitted value of TWO for trinary bit 1 is interpreted as SIX; etc. (For the last trinary bit in the trinary sequence [bit 8], a transmitted value of ONE is interpreted as 6561, and a transmitted value of TWO is interpreted as 13,122). As an example, to transmit a permissible code sequence of 13, 762, the sequence of transmitted values, beginning with the least significant bit, is ONE, ZERO, TWO, TWO, ONE, TWO, ZERO, ZERO, TWO.
The binary code values are encoded on a single frequency carrier in the following manner. A zero is signified by a short carrier burst followed by a short carrier burst; a one is signified by a long carrier burst followed by a long carrier burst; and a two is signified by a long carrier burst followed by a short carrier burst. The timing and positioning of the carrier bursts follow precisely the timing and sequencing constraints noted above. Each trinary sequence is separated from the next by a guard band consisting of a time period during which the carrier is inactive.
An AVI receiver determines the numerical value of a valid received code by adding the values for the trinary bit sent at each bit position in the trinary sequence using the weighting factors noted above. In order to validate the reception of a permissible code sequence, known AVI receivers are designed to require that the identical code sequence be decoded from two or more successive received trinary sequences.
In many loop locations, ambient electro-magnetic radiation can be present, either continuously or sporadically. This radiation is usually referred to as electrical noise signals, or simply noise signals. Some of these noise signals can contain a frequency component having the same frequency as the frequency of the AVI carrier signal generated by AVI transmitters. Given the precise timing constraints resident in the standard AVI trinary encoding process, the presence of such ambient noise signals at the AVI carrier frequency can adversely affect the operation of the AVI detection system, since the AVI receiver must be configured to detect all signals at the predetermined specific frequency. If present at a given loop, these carrier frequency noise signals will pass through the AVI receiver processing circuitry (since it must accept signals at the carrier frequency). The AVI receiver will attempt to process these noise signals, usually with a negative result—e.g., no comparison match with a permissible code sequence. These carrier frequency noise signals can possess sufficient amplitude to mask a permissible code sequence encoded in the carrier frequency. When the carrier frequency noise signals appear at the receiver during the same time as the carrier frequency signals, the AVI system cannot detect and take appropriate action in response to the arrival of an authorized vehicle at the loop. In the case of a fire truck responding to an emergency call in a gate-guarded community, for example, the AVI receiver can fail to generate the necessary gate operating control signal, thus denying the fire truck immediate access to the secured area. In the case of a bus route monitoring application, the AVI receiver can fail to detect the passage of a particular bus, resulting in the loss of important bus location information.
Efforts to devise an AVI system devoid of the above noted disadvantages have not met with success to date.